PROVENANCE OF COBBLES IN THE ROCKY GULCH SANDSTONE MEMBER OF THE HORNBROOK FORMATION (UPPER CRETACEOUS) IN SISKIYOU COUNTY, CALIFORNIA

The Hornbrook Formation (Upper Cretaceous) consists of 1,200 m of marine mudrock, sandstone, and conglomerate deposited in a forearc basin. The Hornbrook is exposed along the northeastern flank of the Klamath Mountains as a homoclinal structure that extends for 80 km from the southwest to the northeast in southwest Oregon and northern California. The Hornbrook Formation is subdivided into five members, in ascending order: Klamath River Conglomerate, Osburger Gulch Sandstone, Ditch Creek Siltstone, Rocky Gulch Sandstone, and Blue Gulch Mudstone. The purpose of this study is to determine the provenance of pebbles and cobbles within conglomerates of the Rocky Gulch Sandstone Member of the Hornbrook Formation.

Conglomerates in the Rocky Gulch are matrix- to grain-supported with thicknesses varying from 0.1 to 20 m, and laterally present for 100s of meters. These conglomerates exhibit normal grading, and their basal contact is gradational to erosional. Sole marks (e.g. flute casts) on sandstones above and below these conglomerates provide northeastern paleocurrents. Pebbles and cobbles in these conglomerates are rounded to well rounded and typically range in size from 0.5 to 25 cm. Approximately 100 pebbles and cobbles collected from an exposure of the Rocky Gulch Sandstone were used in this study. Thin-sections were prepared for selected pebbles and cobbles to better constrain their lithology.

Pebbles and cobbles from the Rocky Gulch are composed primarily of quartzite, felsic to intermediate metavolcanic rocks, and chert with subordinate amounts of metasedimentary rocks and granodiorite. The chert, metavolcanic, and metasedimentary clasts may have been derived from parent materials in the Hayfork and Eastern Klamath terranes in the Klamath Mountains. The granodiorite clasts have a similar composition to the granodiorite of the Jurassic Mt. Ashland pluton in the Klamaths. The source of the quartzite cobbles has not been determined in this study. Overall, the provenance of cobbles in the Rocky Gulch Sandstone is consistent with derivation from the Klamath Mountains. The size of the cobbles also attests to locally derived sediments to the Hornbrook Basin, indicating that the area to the west (a.k.a. Klamath Mountains) was uplifted and a source of sediment in the mid-Cretaceous of southern Oregon.